The present invention relates generally to implantable cardiac pacemakers, and more particularly to a pacemaker which is responsive to a change in the position or posture of the patient to stimulate the patient's heart at a pacing rate which is appropriate to that position.
In recent years a number of intrinsic and extrinsic parameters which undergo change according to states of rest and of physical exercise by the individual have been proposed for use in controlling the rate at which the individual's heart is to be stimulated by an implanted artificial cardiac pacemaker. The intent is that the patient's heart rate should be adapted to the state of true exercise (or rest) of the patient, simulating the intrinsic heart rate of a healthy person with a normal functioning heart who is undergoing the same conditions of exercise or rest.
Most recently, exercise-responsive pacemakers have been advanced and developed which take advantage of simple and inexpensive activity or motion sensors for control of pacing rate. It was suggested several years ago to convert mechanical forces, accelerations and pressures into electrical energy and/or signals for use in biomedical technology. One of the earliest techniques proposed in the patent literature was to generate electrical energy from piezoelectric crystals and other mechanoelectrical converters responsive to movement of the individual to power a device implanted in the individual, as disclosed, for example, in U.S. Pat. Nos. 3,659,615 and 3,456,134. In Journal Biomedizinische Technik 20, pp. 225-228 (1975), Funke described the use of a piezoelectric crystal embedded in silicone rubber and implanted in the pleural space between lung and ribs to detect respiratory rate, for controlling the pacing rate of the patient. U.S. Pat. No. 4,428,380 described using a piezoelectric sensor to measure cardiac activity.
Dahl may have been the first to disclose, in U.S. Pat. No. 4,140,132, the technique of detecting patient activity with a mechanoelectrical converter for the purpose of controlling the rate of a cardiac pacemaker. In Dahl's system, a weighted cantilever arm comprising a piezoelectric crystal is implanted in the patient, the patient's movements cause the cantilever arm to vibrate, the mechanical vibrations are converted to an electrical output signal by the crystal, and the output signal is used as a drive signal for the variable rate pulse generator of the pacemaker. Anderson described a similar system in U.S. Pat. No. 4,428,378 (the "'378 patent"), and used the amplitude of the high frequency content of the converter output signal which was purported to increase with patient movement, as a bandpass signal to control the stimulation rate in an activity-responsive cardiac pacemaker.
Devices such as activity or motion sensors have the distinct advantage that they provide virtually immediate response to patient movements or external forces to generate electrical signals for use in controlling the stimulation pulse rate of the implanted pacemaker. However, they have exhibited serious disadvantages, such as the adverse effect of noise disturbances external to the body, from nearby operating machinery, for example, or emanating from within the body, such as coughing, sneezing, laughing, or the like. Such disturbances are unrelated to physical exercise, but affected the heart rate when early accelerometer-type detectors were utilized for control of the pacemaker stimulation rate. The '378 patent and other prior art sources, such as Proceedings of the European Symposium on Cardiac Pacing, editorial Group, pp. 786 to 790, Madrid, 1985, and Biomedizinische Technik, 4, pp. 79 to 84, 1986, assumed that the maximum acceleration values detected by an activity-controlled cardiac pacemaker in a patient undergoing exercise occur in the range of the resonant frequency of the major body compartments such as the thorax and the abdomen, at approximately 10 Hz (hertz), and that the maximum sensitivity should be in the range above 10 Hz.
In U.S. Pat. No. 4,926,863 (the "'863 patent"), the applicant herein teaches that detection of the accelerometer or activity signal in a frequency range below 10 Hz, and indeed, preferably below approximately 4 Hz is actually highly indicative of true physical exercise by the patient. Moreover, restriction of detection signal frequencies to that range discriminates against and avoids undesirable response to disturbances external and internal to the body. As a result, the effect of disturbances unrelated to exercise can be significantly suppressed during use of a mechanoelectrical converter or like transducer to control the pacing rate.
The '863 patent observes that the amplitude maxima of activity-sensed signals arising from exercise such as walking, climbing stairs, running and bicycling occur with rhythmic motion of the body in the low-frequency range below 10 Hz and principally below 4 Hz. In contrast, sudden spasmodic movements unrelated to true metabolic exercise produce amplitude maxima in the higher-frequency range, well above 4 Hz. Accordingly, the effects of the latter movements, as well as noise disturbances, can be excluded by limiting detection to only the low-frequency content.
By using the frequency band below 10 Hz (preferably below approximately 4 Hz) and by establishing different baseline values as ongoing levels of comparison, the activity pacemaker disclosed in the '863 patent provides fast response and reliable pacing at a variable rate adapted to the level of physical exertion of the patient, closely corresponding to the heart rate of a normal healthy person under the same conditions of physical exertion. Also, by using relative changes rather than absolute values of amplitude of the activity signal to adjust stimulation rate, rate increases are a function of whether a particular baseline value is exceeded and of the actual rate at that time, resulting in smaller rate increases at the higher absolute rates.
In U.S. Pat. No. 5,031,615 (the "'615 patent"), which is a continuation of the '863 patent, the applicant herein discloses an accelerometer and related processing circuitry which are fabricated in hybrid semiconductor integrated circuit form. The accelerometer is designed in that form as a microminiature mechanoelectrical converter or transducer of suitably low power consumption which, as a consequence of its own construction or of use of associated filter circuitry, provides low pass filtering in a frequency band below 10 Hz and preferably below about 4 Hz.
The '863 and '615 patents are incorporated herein in their entirety by reference.
In a copending U.S. patent application Ser. No. 07/863,093, hereinafter referred to as '093 application" of the applicant herein, filed on the same date as the instant application and assigned to the same assignee, an activity pacemaker is programmed to provide different response rates for different types of physical activity of the patient, such as walking, running and bicycling, based on an algorithmic curve which represents the desired and physiologically appropriate heart rates relative to acceleration force. Each type of activity is represented either by a distinct and different portion of the curve of heart rate versus acceleration force with a transition rate between the different portions, or by separate curves which represent distinct and different ones of the activity types. In this way, the patient's heart rate is adapted to different types of activity which, although they may involve the same workload, can have different demands on the patient's cardiovascular system.
These advances and refinements in cardiac pacing have brought about a simple and effective device which is capable of tracking the patient's physical activity constituting true exercise and of controlling the pacing rate of the implanted pacemaker to meet the physiological needs of the patient. To date, however, no device has been suggested which is effective to accommodate the needs of the pacemaker patient's cardiovascular system according to the patient's specific static physical position or posture of the patient (other than a single resting rate), or a change from one such position to another.
It is a principal object of the present invention to provide apparatus and methods for additionally controlling the stimulation rate of an activity pacemaker so that the proper rate is generated in response to the particular static physical position or posture of the patient, such as standing, reclining and lying down, and in response to changes from one position to another.
When a healthy individual with a normal cardiovascular system gets up from a lying or reclining position to a standing position, his or her heart rate increases. If that change of position is followed immediately by some additional activity such as walking, the heart rate may remain at the elevated level or, depending on the level of the activity, increase further. But if the individual simply continues to stand after having arisen, the heart rate more gradually decreases to the resting rate. And if the individual then returns to and remains in a lying position, the heart rate will further decrease to the resting rate characteristic of that position.
It is therefore another important object of the present invention to provide an implantable activity-type pacemaker which detects different physical positions of the patient and responds by generating a pacing rate which is physiologically appropriate for the specific position, each position having its own associated stimulation rate, and by effecting a transition in rates which is appropriate for the change from one position to another whether or not further activity ensues.